Thick film silver paste containing Al2O3 and lead-tellurium—oxide and its use in the manufacture of semiconductor devices

ABSTRACT

The present invention is directed to a thick film silver paste comprising (i) silver, (ii) Al 2 O 3 , and (iii) a Pb—Te—O all dispersed in an organic medium. The present invention is further directed to an electrode formed from the paste and a semiconductor device and, in particular, a solar cell comprising such an electrode. The electrodes provide good adhesion and good electrical performance.

FIELD OF THE INVENTION

The present invention is directed primarily to a thick film silver pasteand electrodes formed from the thick film silver paste. It is furtherdirected to a silicon semiconductor device and, in particular, itpertains to the use of the thick film paste in the formation of anelectrode of a solar cell.

TECHNICAL BACKGROUND OF THE INVENTION

A conventional solar cell structure with a p-type base has a negativeelectrode that is typically on the front-side or sun side of the celland a positive electrode on the back side. Radiation of an appropriatewavelength falling on a p-n junction of a semiconductor body serves as asource of external energy to generate electron-hole pairs in that body.Because of the potential difference which exists at a p-n junction,holes and electrons move across the junction in opposite directions andthereby give rise to a flow of electric current that is capable ofdelivering power to an external circuit. Most solar cells are in theform of a silicon wafer that has been metallized, i.e., provided withmetal electrodes that are electrically conductive. Typically thick filmpastes or inks (referred to simply as “pastes” hereafter) arescreen-printed onto the substrate and fired to form the electrodes.

The front or sun side of the silicon wafer is often coated with ananti-reflective coating (ARC) to prevent reflective loss of incomingsunlight, thus increasing the efficiency of the solar cell. Typically, atwo-dimensional electrode grid pattern, i.e. “front electrode,” makes aconnection to the n-side of the silicon, and a coating of aluminum onthe opposite side (back electrode) makes connection to the p-side of thesilicon. These contacts are the electrical outlets from the p-n junctionto the outside load.

The front electrodes of silicon solar cells are generally formed byscreen-printing a paste. Typically, the paste contains electricallyconductive particles, glass frit and an organic medium. Afterscreen-printing, the wafer and paste are dried at 150° C. for a fewminutes and then fired in air, typically at furnace setpointtemperatures of about 650-1000° C. for a few seconds to form a densesolid of electrically conductive traces. The organic components areburned away in this firing step. Also during this firing step, the glassfrit and any added flux reacts with and etches through theanti-reflective coating and facilitates the formation of intimatesilicon-electrode contact. The glass frit and any added flux alsoprovide adhesion to the substrate and aid in the adhesion ofsubsequently soldered leads to the electrode. Good adhesion to thesubstrate and high solder adhesion of the leads to the electrode areimportant to the performance of the solar cell as well as themanufacturability and reliability of the solar modules.

There is an on-going effort to provide paste compositions that result inelectrodes with improved adhesion and reduced silver content whilemaintaining electrical performance.

SUMMARY OF THE INVENTION

The present invention provides a thick film silver paste comprising:

-   -   (a) silver;    -   (b) Al₂O₃;    -   (c) a Pb—Te—O; and    -   (d) organic medium;        wherein the silver, the Al₂O₁, and the Pb—Te—O are dispersed in        the organic medium.

The thick film silver paste contains 50 to 95 wt % inorganic solids,i.e., the total content of the silver, the Al₂O₃ and the Pb—Te—O is 50to 95 wt %, based on the total weight of the paste.

The invention also provides a semiconductor device, and in particular, asolar cell, comprising an electrode formed from the instant paste,wherein the paste has been fired to remove the organic medium and formthe electrode. In an embodiment the electrode is a front electrode, inanother embodiment the electrode is a back electrode, e.g., a tabbingelectrode.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1F illustrate the fabrication of a semiconductor device.

Reference numerals shown in FIG. 1 are explained below,

-   -   10: p-type silicon substrate    -   20: n-type diffusion layer    -   30: ARC (e.g., silicon nitride film, titanium oxide film, or        silicon oxide film)    -   40: p+ layer (back surface field, BSF)    -   60: aluminum paste deposited on back side    -   61: aluminum back side electrode (obtained by firing back side        aluminum paste)    -   70: silver/aluminum paste deposited on back side    -   71: silver/aluminum back side electrode (obtained by firing back        side silver/aluminum paste)    -   500: paste of the instant invention deposited on front side    -   501: front electrode (formed by firing front side paste 500)

DETAILED DESCRIPTION OF THE INVENTION

The thick film silver paste of the instant invention simultaneouslyprovides the ability to form an electrode wherein the electrode hasreduced cost because of the reduced amount of silver used but alsoexhibits good electrical and improved adhesion properties. The thickfilm silver paste can be printed or applied with the desired pattern,such as by screen-printing, plating, ink-jet printing, extrusion, shapedor multiple printing, or ribbons.

The thick film silver paste comprises silver, Al₂O₃; a Pb—Te—O, and anorganic medium. In one embodiment, the composition comprises 50-95 vol %silver and 5-50 vol % Al₂O₃, wherein the vol % are based on the totalvolume of the silver and the Al₂O₃.

The thick film silver paste of the instant invention provides electrodeswith improved adhesion and reduced silver content while maintainingelectrical performance.

Each constituent of the thick film silver paste of the present inventionis discussed in detail below.

Silver

The silver (Ag) can be in the form of silver metal, alloys of silver, ormixtures thereof. Typically, in a silver powder, the silver particlesare in a flake form, a spherical form, a granular form, a crystallineform, other irregular forms and mixtures thereof. The silver can beprovided in a colloidal suspension. The silver can also be in the formof silver resonates (organometallic silver), silver oxide (Ag₂O), silversalts such as AgCl, AgNO₃, AgOOCCH₃ (silver acetate), AgOOCF₃ (silvertrifluoroacetate), silver orthophosphate (Ag₃PO₄), or mixtures thereof.Other forms of silver compatible with the other constituents can also beused.

In one embodiment, the thick film paste comprises 50-95 vol % silver,wherein the vol % is based on the total volume of the silver and theAl₂O₃. In another embodiment, the thick film paste comprises 70-90 vol %silver, wherein the vol % is based on the total volume of the silver andthe Al₂O₃. In still another embodiment, the thick film paste comprises50-60 vol % silver, wherein the vol % is based on the total volume ofthe silver and the Al₂O₃.

In one embodiment, the thick film paste comprises coated silverparticles that are electrically conductive. Suitable coatings includesurfactants and phosphorous-containing compounds. Suitable surfactantsinclude polyethyleneoxide, polyethyleneglycol, benzotriazole,poly(ethyleneglycol)acetic acid, lauric acid, oleic acid, capric acid,myristic acid, linolic acid, stearic acid, palmitic acid, stearatesalts, palmitate salts, and mixtures thereof. The salt counter-ions canbe ammonium, sodium, potassium, and mixtures thereof.

The particle size of the silver is not subject to any particularlimitation. In one embodiment, the average particle size is less than 10microns; in another embodiment, the average particle size is in therange of 1 to 6 microns.

Al₂O₃

The Al₂O₃ (aluminum oxide) is in the form of a powder with granularparticles. Typically, the average particle size is less than 10 microns.In one embodiment, the particle size distribution of the powder is suchthat d₅₀ is between 5 and 6 microns.

In an embodiment, the thick film paste comprises 5-50 vol % Al₂O₃,wherein the vol % is based on the total volume of the silver and theAl₂O₃. In another embodiment, the thick film paste comprises 10-30 vol %Al₂O₃, wherein the vol % is based on the total volume of the silver andthe Al₂O₃. In still another embodiment, the thick film paste comprises40-50 vol % Al₂O₃, wherein the vol % is based on the total volume of thesilver and the Al₂O₃.

Lead-Tellurium-Oxide

A component of the paste is a lead-tellurium-oxide (Pb—Te—O). In anembodiment, this oxide may be a glass composition, e.g., a glass frit.In a further embodiment, this oxide may be crystalline, partiallycrystalline, amorphous, partially amorphous, or combinations thereof. Inan embodiment, the Pb—Te—O may include more than one glass composition.In an embodiment, the Pb—Te—O composition may include a glasscomposition and an additional composition, such as a crystallinecomposition.

The lead-tellurium-oxide (Pb—Te—O) may be prepared by mixing TeO₂, leadoxide and other oxides to be incorporated therein (or other materialsthat decompose into the desired oxides when heated) using techniquesunderstood by one of ordinary skill in the art. The lead oxide mayinclude one or more components selected from the group consisting ofPbO, Pb₃O₄, and PbO₂. Such preparation techniques may involve heatingthe mixture in air or an oxygen-containing atmosphere to form a melt,quenching the melt, and grinding, milling, and/or screening the quenchedmaterial to provide a powder with the desired particle size. Melting themixture of lead, tellurium and other oxides to be incorporated thereinis typically conducted to a peak temperature of 800 to 1200° C. Themolten mixture can be quenched, for example, on a stainless steel platenor between counter-rotating stainless steel rollers to form a platelet.The resulting platelet can be milled to form a powder. Typically, themilled powder has a d₅₀ of 0.1 to 3.0 microns. One skilled in the art ofproducing glass frit may employ alternative synthesis techniques such asbut not limited to water quenching, sol-gel, spray pyrolysis, or othersappropriate for making powder forms of glass.

The oxide product of the above process is typically essentially anamorphous (non-crystalline) solid material, i.e., a glass. However, insome embodiments the resulting oxide may be amorphous, partiallyamorphous, partially crystalline, crystalline or combinations thereof.As used herein “glass frit” includes all such products.

Glass compositions, also termed glass frits, are described herein asincluding percentages of certain components. Specifically, thepercentages are the percentages of the components used in the startingmaterial that was subsequently processed as described herein to form aglass composition. Such nomenclature is conventional to one of skill inthe art. In other words, the composition contains certain components,and the percentages of those components are expressed as a percentage ofthe corresponding oxide form. As recognized by one of ordinary skill inthe art in glass chemistry, a certain portion of volatile species may bereleased during the process of making the glass. An example of avolatile species is oxygen. It should also be recognized that while theglass behaves as an amorphous material it will likely contain minorportions of a crystalline material.

If starting with a fired glass, one of ordinary skill in the art maycalculate the percentages of starting components described herein usingmethods known to one of skill in the art including, but not limited to:Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), InductivelyCoupled Plasma-Atomic Emission Spectroscopy (ICP-AES), and the like. Inaddition, the following exemplary techniques may be used: X-RayFluorescence spectroscopy (XRF); Nuclear Magnetic Resonance spectroscopy(NMR); Electron Paramagnetic Resonance spectroscopy (EPR); Mössbauerspectroscopy; electron microprobe Energy Dispersive Spectroscopy (EDS);electron microprobe Wavelength Dispersive Spectroscopy (WDS); orCathodo-Luminescence (CL).

One of ordinary skill in the art would recognize that the choice of rawmaterials could unintentionally include impurities that may beincorporated into the glass during processing. For example, theimpurities may be present in the range of hundreds to thousands ppm. Thepresence of the impurities would not alter the properties of the glass,the composition, e.g. a thick film composition, or the fired device. Forexample, a solar cell containing a thick film composition may have theefficiency described herein, even if the thick film composition includesimpurities.

Typically, the mixture of PbO and TeO₂ powders used to make the Pb—Te—Oincludes 5 to 95 mol % of lead oxide and 5 to 95 mol % of telluriumoxide, based on the combined powders. In one embodiment, the mixture ofPbO and TeO₂ powders includes 25 to 85 mol % of lead oxide and 15 to 75mol % of tellurium oxide, based on the combined powders. In anotherembodiment, the mixture of PbO and TeO₂ powders includes 25 to 65 mol %of lead oxide and 35 to 75 mol % of tellurium oxide, based on thecombined powders.

In one embodiment, the electrively conductive composition comprises0.5-10 wt % Pb—Te—O, wherein said wt % is based on the total weight ofthe thick film paste. In another embodiment, the thick film pastecomprises 0.5-5 wt % Pb—Te—O, wherein said wt % is based on the totalweight of said composition.

In some embodiments, the mixture of PbO and TeO₂ powders furtherincludes one or more other metal compounds. Suitable other metalcompounds include TiO₂, LiO₂, B₂O₃, PbF₂, SiO₂, Na₂O, K₂O, Rb₂O, Cs₂O,Al₂O₃, MgO, CaO, SrO, BaO, V₂O₅, ZrO₂, MoO₃, Mn₂O₃, Ag₂O, ZnO, Ga₂O₃,GeO₂, In₂O₃, SnO₂, Sb₂O₃, Bi₂O₃, BiF₃, P₂O₅, CuO, NiO, Cr₂O₃, Fe₂O₃,CoO, Co₂O₃, and CeO₂. Table 1 lists some examples of powder mixturescontaining PbO, TeO₂ and other optional metal compounds that can be usedto make lead-tellurium oxides. This list is meant to be illustrative,not limiting. In Table 1, the amounts of the compounds are shown as wt%, based on the weight of the total glass composition.

Therefore as used herein, the term “Pb—Te—O” may also include metaloxides that contain oxides of one or more elements selected from thegroup consisting of Si, Sn, Li, Ti, B, Ag, Na, K, Rb, Cs, Ge, Ga, In,Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy,Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, F, Zr, Mn, P, Cu, Ce, andNb.

TABLE 1 Powder Wt % Wt % Wt % Wt % Wt % Wt % Wt % Wt % Wt % mixture PbOTeO₂ PbF₂ SiO₂ B₂O₃ P₂O₅ SnO₂ Ag₂O Li₂O A 32.95 67.05 B 38.23 51.2610.50 C 67.72 32.28 D 72.20 27.80 E 80.75 19.25 F 59.69 9.30 16.19 14.82G 75.86 9.26 14.88 H 48.06 51.55 0.39 I 48.16 51.65 0.19 J 47.44 50.881.68 K 47.85 51.33 0.82 L 41.76 44.80 0.32 0.80 12.32 M 46.71 50.10 3.19N 46.41 49.78 3.80 O 45.11 48.39 6.50 P 44.53 47.76 7.71 Q 48.05 51.540.41 R 47.85 51.33 0.82 S 47.26 50.70 2.04 T 45.82 49.19 U 48.04 51.53 V39.53 28.26 W 48.04 51.53 0.42

In one embodiment, the Pb—Te—O includes boron, i.e., the Pb—Te—O isPb—Te—B—O. The starting mixture used to make the Pb—Te—B—O may include(based on the weight of the total starting mixture) PbO that may be 25to 75 wt %, 30 to 60 wt %, or 30 to 50 wt %; TeO₂ that may be 10 to 70wt %, 25 to 60 wt %, or 40 to 60 wt %; B₂O₃ that may be 0.1 to 15 wt %,0.25 to 5 wt %, or 0.4 to 2 wt %.

In an embodiment, PbO, TeO₂, and B₂O₃ may be 80-100 wt % of thePb—Te—B—O composition. In a further embodiment, PbO, TeO₂, and B₂O₃ maybe 85-100 wt % or 90-100 wt % of the Pb—Te—B—O composition.

In a further embodiment, in addition to the above PbO, TeO₂, and B₂O₃,the starting mixture used to make the Pb—Te—B—O may include one or moreof PbF₂, SiO₂, BiF₃, SnO₂, Li₂O, Bi₂O₃, ZnO, V₂O₅, Na₂O, TiO₂, Al₂O₃,CuO, ZrO₂, CeO₂, or Ag₂O. In an embodiment, one or more of thesecomponents may be 0-20 wt %, 0-15 wt %, or 0-10 wt % of the Pb—Te—B—Ocomposition. In aspects of this embodiment (based on the weight of thetotal starting mixture):

the PbF₂ may be 0 to 20 wt %, 0 to 15 wt %, or 5 to 10 wt %;

the SiO₂ may be 0 to 11 wt %, 0 to 5 wt %, 0.25 to 4 wt %, or 0 to 0.5wt %;

the BiF₃ may be 0 to 15 wt %, 0 to 10 wt %, or 1 to 10 wt %;

the SnO₂ may be 0 to 5 wt %, 0 to 2 wt %, or 0.5 to 1.5 wt %;

the ZnO may be 0 to 5 wt %, 0 to 3 wt %, or 2 to 3 wt %;

the V₂O₅ may be 0 to 5 wt %, 0 to 1 wt %, or 0.5 to 1 wt %;

the Na₂O may be 0 to 5 wt %, 0 to 3 wt %, or 0.1 to 1.5 wt %;

the CuO may be 0 to 5 wt %, 0 to 3 wt %, or 2 to 3 wt %;

the ZrO₂ may be 0 to 3 wt %, 0 to 2 wt %, or 0.1 to 1 wt %;

the CeO₂ may be 0 to 5 wt %, 0 to 3 wt %, or 0.1 to 2.5 wt %;

the Li₂O may be 0 to 5 wt %, 0.1 to 3 wt %, or 0.25 to 2 wt %;

the Bi₂O₃ may be 0 to 15 wt %, 0 to 10 wt %, or 5 to 8 wt %;

the TiO₂ may be 0 to 5 wt %, 0.25 to 5 wt %, or 0.25 to 2.5 wt %;

the Al₂O₃ may be 0 to 3 wt %, 0 to 2 wt %, or 0.1 to 2 wt %; and

the Ag₂O may be 0 to 10 wt %, 1 to 10 wt %, or 1 to 8 wt %.

In an embodiment, the Pb—Te—B—O may be a homogenous powder. In a furtherembodiment, the Pb—Te—B—O may be a combination of more than one powder,wherein each powder may separately be a homogenous population. Thecomposition of the overall combination of the multiple powders is withinthe ranges described above. For example, the Pb—Te—B—O may include acombination of two or more different powders; separately, these powdersmay have different compositions, and may or may not be within the rangesdescribed above; however, the combination of these powders is within theranges described above.

In an embodiment, the Pb—Te—B—O composition may include one powder whichincludes a homogenous powder including some but not all of the elementsof the group Pb, Te, B, and O, and a second powder, which includes oneor more of the elements of the group Pb, Te, B, and O. For example, thePb—Te—B—O composition may include a first powder including Pb, Te, andO, and a second powder including B₂O₃. In an aspect of this embodiment,the powders may be melted together to form a uniform composition. In afurther aspect of this embodiment, the powders may be added separatelyto a thick film composition.

In an embodiment, some or all of the Li₂O may be replaced with Na₂O,K₂O, Cs₂O, or Rb₂O, resulting in a glass composition with propertiessimilar to the compositions listed above. In this embodiment, the totalalkali metal oxide content may be 0 to 5 wt %, 0.1 to 3 wt %, or 0.25 to3 wt %.

In a further embodiment, the Pb—Te—B—O composition(s) herein may includeone or more of a third set of components: GeO₂, Ga₂O₃, NiO, CoO, ZnO,CaO, MgO, SrO, MnO, BaO, SeO₂, MoO₃, WO₃, Y₂O₃, As₂O₃, La₂O₃, Nd₂O₃,Bi₂O₃, Ta₂O₅, V₂O₅, FeO, HfO₂, Cr₂O₃, CdO, Sb₂O₃, PbF₂, ZrO₂, Mn₂O₃,P₂O₅, CuO, Pr₂O₃, Gd₂O₃, Sm₂O₃, Dy₂O₃, Eu₂O₃, Ho₂O₃, Yb₂O₃, Lu₂O₃, CeO₂,BiF₃, SnO, SiO₂, Ag₂O, Nb₂O₅, TiO₂, Rb₂O, SiO₂, Na₂O, K₂O, Cs₂O, Lu₂O₃,SnO₂, and metal halides (e.g., NaCl, KBr, NaI, LiF, ZnF₂).

Therefore as used herein, the term “Pb—Te—B—O” may also include metaloxides that contain one or more oxides of elements selected from thegroup consisting of Si, Sn, Li, Ti, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni,Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy, Eu,Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, F, Zr, Mn, P, Cu, Ce, and Nb.

In another embodiment, the Pb—Te—O includes lithium, i.e., the Pb—Te—Ois Pb—Te—Li—O. The starting mixture used to make the Pb—Te—Li—O mayinclude (based on the weight of the total starting mixture): PbO thatmay be 30 to 60 wt %, 40 to 55 wt %, or 45 to 50 wt %; TeO₂ that may be40 to 65 wt %, 45 to 60 wt %, or 50 to 55 wt %; and Li₂O that may be 0.1to 5 wt %, 0.2 to 3 wt %, or 0.3 to 1 wt %.

In a further embodiment, in addition to the above PbO, TeO₂, and Li₂O,the starting mixture used to make the Pb—Te—Li—O may include one or moreof SiO₂, SnO₂, B₂O₃, Ag₂O, BiF₃, V₂O₅, Na₂O, ZrO₂, TiO₂, CeO₂, Bi₂O₃,Nb₂O₅, Ta₂O₅, K₂O, MgO, P₂O₅, SeO₂, CO₃O₄, PdO, RuO₂, NiO, ZnO, CuO,MnO, Cr₂O₃, or Al₂O₃. In aspects of this embodiment (based on the weightof the total starting mixture):

the SiO₂ may be 0 to 11 wt %, 0 to 5 wt %, 0.25 to 4 wt %, or 0 to 0.5wt %;

the SnO₂ may be 0 to 5 wt %, 0 to 2 wt %, or 0.5 to 1.5 wt %;

the B₂O₃ may be 0 to 10 wt %, 0 to 5 wt %, or 0.5 to 5 wt %;

the Ag₂O may be 0 to 30 wt %, 0 to 20 wt %, 3 to 15 wt % or 1 to 8 wt %;

the TiO₂ may be 0 to 5 wt %, 0.25 to 5 wt %, or 0.25 to 2.5 wt %;

the PbF₂ may be 0 to 20 wt %, 0 to 15 wt %, or 5 to 10 wt %;

the BiF₃ may be 0 to 15 wt %, 0 to 10 wt %, or 1 to 10 wt %;

the ZnO may be 0 to 5 wt %, 0 to 3 wt %, or 2 to 3 wt %;

the V₂O₅ may be 0 to 5 wt %, 0 to 1 wt %, or 0.5 to 1 wt %;

the Na₂O may be 0 to 5 wt %, 0 to 3 wt %, or 0.1 to 1.5 wt %;

the CuO may be 0 to 5 wt %, 0 to 3 wt %, or 2 to 3 wt %;

the ZrO₂ may be 0 to 3 wt %, 0 to 2 wt %, or 0.1 to 1 wt %;

the CeO₂ may be 0 to 5 wt %, 0 to 3 wt %, or 0.1 to 2.5 wt %;

the Bi₂O₃ may be 0 to 15 wt %, 0 to 10 wt %, or 5 to 8 wt %; and

the Al₂O₃ may be 0 to 3 wt %, 0 to 2 wt %, or 0.1 to 2 wt %.

In one such embodiment, in addition to the above PbO, TeO₂, and Li₂O,the starting mixture used to make the Pb—Te—Li—O includes B₂O₃ and BiF₃,or a mixture of Bi₂O₃ and BiF₃. In this embodiment, the Pb—Te—Li—O isPb—Te—Li—B—Bi—O.

In an embodiment, the Pb—Te—Li—O may be a homogenous powder. In afurther embodiment, the Pb—Te—Li—O may be a combination of more than onepowder, wherein each powder may separately be a homogenous population.The composition of the overall combination of the two powders may bewithin the ranges described above. For example, the Pb—Te—Li—O mayinclude a combination of two or more different powders; separately,these powders may have different compositions; and may or may not bewithin the ranges described above; however, the combination of thesepowders may be within the ranges described above.

In an embodiment, the Pb—Te—Li—O composition may include one powderwhich includes a homogenous powder including some but not all of theelements of the group Pb, Te, Li, and O, and a second powder, whichincludes one or more of the elements of the group Pb, Te, Li, and O,

In an embodiment, some or all of the Li₂O may be replaced with Na₂O,K₂O, Cs₂O, or Rb₂O, resulting in a glass composition with propertiessimilar to the compositions listed above. In this embodiment, the totalalkali metal oxide content may be 0 to 5 wt %, 0.1 to 3 wt %, or 0.25 to3 wt %.

In a further embodiment, the glass frit composition(s) herein mayinclude one or more of a third set of components: GeO₂, Ga₂O₃, NiO, CoO,ZnO, CaO, MgO, SrO, MnO, BaO, SeO₂, MoO₃, WO₃, Y₂O₃, As₂O₃, La₂O₃,Nd₂O₃, Bi₂O₃, B₂O₃, Ta₂O₅, V₂O₅, FeO, HfO₂, Cr₂O₃, CdO, Sb₂O₃, PbF₂,ZrO₂, Mn₂O₃, P₂O₅, CuO, La₂O₃, Pr₂O₃, Nd₂O₃, Gd₂O₃, Sm₂O₃, Dy₂O₃, Eu₂O₃,Ho₂O₃, Yb₂O₃, Lu₂O₃, CeO₂, BiF₃, SnO, SiO₂, Ag₂O, Nb₂O₅, TiO₂, and metalhalides (e.g., NaCl, KBr, NaI, LiF).

Therefore as used herein, the term “Pb—Te—Li—O” may also include metaloxides that contain one or more elements selected from the groupconsisting of Si, Sn, Ti, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni, Zn, Ca, Mg,Sr, Ba, Se, Mo, W, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu,Bi, B, Ta, V. Fe, Hf, Cr, Cd, Sb, F, Zr, Mn, P, Cu, Ce, and Nb.

Table 2 lists some examples of powder mixtures containing PbO, TeO₂,Li₂O that can be used to make lead-tellurium-lithium-oxides. This listis meant to be illustrative, not limiting. In Table 2, the amounts ofthe compounds are shown as wt %, based on the weight of the total glasscomposition.

TABLE 2 Glass # PbO Li₂O TeO₂ 1 48.04 0.42 51.54 2 47.74 1.05 51.21 344.73 0.43 54.84 4 55.49 0.41 44.09 5 58.07 0.41 41.52 6 34.51 2.4463.06 7 42.77 0.43 56.80 8 45.82 4.99 49.19 9 48.04 0.42 51.53 10 47.820.89 51.29 11 42.77 0.43 56.80 12 37.31 0.44 62.25 13 57.80 0.86 41.3314 58.07 0.41 41.52

In still another embodiment, the Pb—Te—O includes lithium and titanium,i.e., the Pb—Te—O is Pb—Te—Li—Ti—O. The starting mixture used to makethe Pb—Te—Li—Ti—O includes, based on the total weight of the startingmixture of the Pb—Te—Li—Ti—O, 25-65 wt % PbO, 25-70 wt % TeO₂, 0.1-5 wt% Li₂O and 0.1-5 wt % TiO₂. In one embodiment, the starting mixture usedto make the Pb—Te—Li—Ti—O includes, based on the total weight of thestarting mixture of the Pb—Te—Li—Ti—O, 30-60 wt % PbO, 30-65 wt % TeO₂,0.25-3 wt % Li₂O and 0.25-5 wt % TiO₂. In another embodiment, thestarting mixture includes 30-50 wt % PbO, 50-65 wt % TeO₂, 0.5-2.5 wt %Li₂O and 0.5-3 wt % TiO₂.

In any of the above embodiments, Pb—O, TeO₂, Li₂O₃, and TiO₂ may be80-100 wt % of the Pb—Te—Li—Ti—O composition. In further embodiments,PbO, TeO₂, Li₂O₃, and TiO₂ may be 85-100 wt % or 90-100 wt % of thePb—Te—Li—Ti—O composition.

In any of the above embodiments, in addition to the above Pb—O, TeO₂,Li₂O, and TiO₂, the Pb—Te—Li—Ti—O further comprises an oxide selectedfrom the group consisting of SiO₂, SnO₂, B₂O₃, ZnO, Nb₂O₅, CeO₂, V₂O₅,Al₂O₃, Ag₂O and mixtures thereof. In aspects of this embodiment (basedon the weight of the total starting mixture);

-   -   the SiO₂ may be 0 to 10 wt %, 0 to 9 wt %, or 2 to 9 wt %;    -   the SnO₂ may be 0 to 5 wt %, 0 to 4 wt %, or 0.5 to 1.5 wt %;    -   the B₂O₃ may be 0 to 10 wt %, 0 to 5 wt %, or 1 to 5 wt %; and    -   the Ag₂O may be 0 to 30 wt %, 0 to 20 wt %, or 3 to 15 wt %.

In addition, in any of the above embodiments, the glass frit compositionherein may include one or more of a third set of components: GeO₂,Ga₂O₃, In₂O₃, NiO, ZnO, CaO, MgO, SrO, BaO, SeO₂, MoO₃, WO₃, Y₂O₃,As₂O₃, La₂O₃, Nd₂O₃, Bi₂O₃, BiF₃, Ta₂O₅, FeO, HfO₂, Cr₂O₃, CdO, Sb₂O₃,PbF₂, ZrO₂, Mn₂O₃, P₂O₅, CuO, Nb₂O₅, Rb₂O, Na₂O, K₂O, Cs₂O, Lu₂O₃, andmetal halides (e.g., NaCl, KBr, NaI, LiF, ZnF₂).

Therefore as used herein, the term “Pb—Te—Li—Ti—O” may also containoxides of one or more elements selected from the group consisting of Si,Sn, B, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, W,Y, As, La, Nd, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, Zr, Mn, P, Cu, Lu, Ce, Aland Nb.

Tables 3 and 4 list some examples of powder mixtures containing PbO,TeO₂, Li₂O, TiO₂, and other optional compounds that can be used to makelead-tellurium-lithium-titanium-oxides. This list is meant to beillustrative, not limiting. In Tables 3 and 4, the amounts of thecompounds are shown as weight percent, based on the weight of the totalPb—Te—Li—Ti—O composition.

The lead-tellurium-lithium-titanium-oxide (Pb—Te—Li—Ti—O) compositionsof Table 3 were prepared by mixing and blending amounts of Pb₃O₄, TeO₂,Li₂CO₃, and TiO₂ powders, and optionally, as shown in Table 3, SiO₂,B₂O₃, Ag₂O, and/or SnO₂ to provide compositions of the oxides with theweight percentages shown in Table 3, based on the weight of the totalglass composition.

TABLE 3 Frit SiO₂ PbO B₂O₃ Li₂O TiO₂ Ag₂O SnO₂ TeO₂ 1 8.40 60.90 1.470.93 0.70 27.60 2 46.04 0.40 4.18 49.38 3 46.78 0.83 2.22 50.17 4 47.430.85 0.84 50.88 5 33.77 2.39 2.13 61.71 6 45.35 0.48 0.43 53.74 7 36.191.99 1.77 60.05 8 37.35 2.39 2.13 58.13 9 36.19 1.82 3.06 58.94 10 40.812.39 2.13 54.67 11 44.28 0.16 0.42 12.29 42.84 12 40.81 0.59 1.57 9.0847.95 13 40.81 1.90 1.12 56.16 14 45.77 1.09 0.80 0.71 51.64 15 41.200.34 2.30 56.16 16 44.31 0.52 0.46 0.96 3.57 50.17 17 42.92 0.54 0.781.31 54.44 18 42.22 0.91 1.53 55.35

The lead-tellurium-lithium-titanium-oxide (Pb—Te—Li—Ti—O) compositionsof Table 4 were prepared by mixing and blending amounts of Pb₃O₄, TeO₂,Li₂CO₃ and TiO₂ powders, and optionally, as shown in Table 4, B₂O₃, ZnO,Nb₂O₅, CeO₂, and/or V₂O₅ to provide compositions of the oxides with theweight percentages shown in Table 4, based on the weight of the totalglass composition,

TABLE 4 Frit PbO B₂O₃ ZnO Nb₂O₅ Li₂O TiO₂ CeO₂ V₂O₅ TeO₂ 19 42.27 0.941.51 2.87 52.40 20 42.57 4.13 0.92 1.54 50.85 21 45.26 0.86 2.25 0.550.49 1.06 49.53

In one embodiment, the Pb—Te—Li—Ti—O may be a homogenous powder. In afurther embodiment, the Pb—Te—Li—Ti—O may be a combination of more thanone powder, wherein each powder may separately be a homogenouspopulation. The composition of the overall combination of the 2 powdersis within the ranges described above. For example, the Pb—Te—Li—Ti—O mayinclude a combination of 2 or more different powders; separately, thesepowders may have different compositions, and may or may not be withinthe ranges described above; however, the combination of these powders iswithin the ranges described above.

In an embodiment, the Pb—Te—Li—Ti—O composition may include one powderwhich includes a homogenous powder including some but not all of thedesired elements of the Pb—Te—Li—Ti—O composition, and a second powder,which includes one or more of the other desired elements. For example, aPb—Te—Li—Ti—O composition may include a first powder including Pb, Te,Li, and O, and a second powder including TiO₂. In an aspect of thisembodiment, the powders may be melted together to form a uniformcomposition. In a further aspect of this embodiment, the powders may beadded separately to a thick film composition.

In an embodiment, some or all of any Li₂O may be replaced with Na₂O,K₂O, Cs₂O, or Rb₂O, resulting in a glass composition with propertiessimilar to the compositions listed above. In this embodiment, the totalalkali metal content will be that described above for Li₂O.

Organic Medium

The inorganic components of the paste are mixed with an organic mediumto form viscous thick film pastes or less viscous inks having suitableconsistency and rheology for printing. A wide variety of inert viscousmaterials can be used as the organic medium. The organic medium can beone in which the inorganic components are dispersible with an adequatedegree of stability during manufacturing, shipping and storage of thepastes or inks, as well as on the printing screen during ascreen-printing process.

Suitable organic media have rheological properties that provide stabledispersion of solids, appropriate viscosity and thixotropy for printing,appropriate wettability of the substrate and the paste solids, a gooddrying rate, and good firing properties. The organic medium can containthickeners, stabilizers, surfactants, and/or other common additives. Onesuch thixotropic thickener is Thixatrol® (Elementis plc, London, UK).The organic medium CaO be a solution of polymer(s) in solvent(s).Suitable polymers include ethyl cellulose, ethylhydroxyethyl cellulose,wood rosin, mixtures of ethyl cellulose and phenolic resins,polymethacrylates of lower alcohols, and the monobutyl ether of ethyleneglycol monoacetate. Suitable solvents include terpenes such as alpha- orbeta-terpineol or mixtures thereof with other solvents such as kerosene,dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexyleneglycol and alcohols with boiling points above 150° C., and alcoholesters. Other suitable organic medium components include:bis(2-(2-butoxyethoxy)ethyl adipate, dibasic esters such as DBE, DBE-2,DBE-3, DBE-4, DBE-5, DBE-6, DBE-9, and DBE 1B, octyl epoxy tallate,isotetradecanol, and pentaerythritol ester of hydrogenated rosin. Theorganic medium can also comprise volatile liquids to promote rapidhardening after application of the paste composition on a substrate.

The optimal amount of organic medium in the paste is dependent on themethod of applying the composition, the specific organic medium used andpurpose fro which the paste is being used. The paste contains 5 to 50 wt% of organic medium, based on the total weight of the composition.

If the organic medium comprises a polymer, the polymer typicallycomprises 8 to 15 wt % of the organic composition.

Preparation of the Paste

In one embodiment, the paste can be prepared by mixing the silver, theAl₂O₃, the Pb—Te—O, and the organic medium in any order. In someembodiments, the inorganic materials are mixed first, and they are thenadded to the organic medium. In other embodiments, the silver, which isthe major portion of the inorganics, is slowly added to the organicmedium, The viscosity can be adjusted, if needed, by the addition ofsolvents. Mixing methods that provide high shear are useful.

Formation of Electrodes

The composition can be deposited, for example, by screen-printing,plating, extrusion, ink-jet printing, shaped or multiple printing, orribbons.

In this electrode-forming process, the composition is first dried andthen heated to remove the organic medium and sinter the inorganicmaterials. The heating can be carried out in air or an oxygen-containingatmosphere. This step is commonly referred to as “firing.” The firingtemperature profile is typically set so as to enable the burnout oforganic binder materials from the dried paste composition, as well asany other organic materials present. In one embodiment, the firingtemperature is 700 to 950° C. The firing can be conducted in a beltfurnace using high transport rates, for example, 100-500 cm/min, withresulting hold-up times of 0.03 to 5 minutes. Multiple temperaturezones, for example 3 to 11 zones, can be used to control the desiredthermal profile.

In one embodiment, a semiconductor device is manufactured from anarticle comprising a junction-bearing semiconductor substrate and asilicon nitride insulating film formed on a main surface thereof. Theinstant composition is applied (e.g., coated or screen-printed) onto theinsulating film, in a predetermined shape and thickness and at apredetermined position. The instant composition has the ability topenetrate the insulating layer. Firing is then carried out and thecomposition reacts with the insulating film and penetrates theinsulating film, thereby effecting electrical contact with the siliconsubstrate and as a result the electrode is formed.

An example of this method of forming the electrode is described below inconjunction with FIGS. 1A-1F.

FIG. 1A shows a single crystal or multi-crystalline p-type siliconsubstrate 10.

In FIG. 1B, an n-type diffusion layer 20 of the reverse conductivitytype is formed by the thermal diffusion of phosphorus using phosphorusoxychloride as the phosphorus source. In the absence of any particularmodifications, the diffusion layer 20 is formed over the entire surfaceof the silicon p-type substrate 10. The depth of the diffusion layer canbe varied by controlling the diffusion temperature and time, and isgenerally formed in a thickness range of about 0.3 to 0.5 microns. Then-type diffusion layer may have a sheet resistivity of several tens ofohms per square up to about 120 ohms per square.

After protecting the front surface of this diffusion layer with a resistor the like, as shown in FIG. 1C the diffusion layer 20 is removed fromthe rest of the surfaces by etching so that it remains only on the frontsurface. The resist is then removed using an organic solvent or thelike.

Then, as shown in FIG. 1D an insulating layer 30 which also functions asan anti-reflection coating (ARC) is formed on the n-type diffusion layer20. The insulating layer is commonly silicon nitride, but can also be aSiN_(x):H film (i.e., the insulating film comprises hydrogen forpassivation during subsequent firing processing), a titanium oxide film,a silicon oxide film, or a silicon oxide/titanium oxide film. Athickness of about 700 to 900 Å of a silicon nitride film is suitablefor a refractive index of about 1.9 to 2.0. Deposition of the insulatinglayer 30 can be by sputtering, chemical vapor deposition, or othermethods.

Next, electrodes are formed. As shown in FIG. 1E, the thick film pasteof the present invention 500 is screen-printed to create the frontelectrode on the insulating film 30 and then dried. In addition, a backside silver or silver/aluminum paste 70, and an aluminum paste 60 arethen screen-printed onto the back side of the substrate and successivelydried. Firing is carried out in an infrared belt furnace at atemperature range of approximately 750 to 950° C. for a period of fromseveral seconds to several tens of minutes.

Consequently, as shown in FIG. 1F, during firing, aluminum diffuses fromthe aluminum paste 60 into the silicon substrate 10 on the back sidethereby forming a p+ layer 40 containing a high concentration ofaluminum dopant. This layer is generally called the back surface field(BSF) layer, and helps to improve the energy conversion efficiency ofthe solar cell.

Firing converts the dried aluminum paste 60 to an aluminum backelectrode 61. The back side silver or silver/aluminum paste 70 is firedat the same time, becoming a silver or silver/aluminum back electrode,71. During firing, the boundary between the back side aluminum and theback side silver or silver/aluminum assumes the state of an alloy,thereby achieving electrical connection. Most areas of the backelectrode are occupied by the aluminum electrode 61, owing in part tothe need to form a p+ layer 40. Because soldering to an aluminumelectrode is impossible, the silver or silver/aluminum back electrode 71is formed over portions of the back side as an electrode forinterconnecting solar cells by means of copper ribbon or the like. Inaddition, the front side thick film paste 500 of the present inventionsinters and penetrates through the insulating film 30 during firing, andthereby achieves electrical contact with the n-type layer 20. This typeof process is generally called “fire through.” The fired electrode 501of FIG. 1F clearly shows the result of the fire through.

EXAMPLES

Solar Cell Electrical Measurements

A commercial Current-Voltage (JV) tester ST-1000 (Telecom-STV Ltd.,Moscow, Russia) was used to make efficiency and fill factor measurementsof the polycrystalline silicon photovoltaic cells. Two electricalconnections, one for voltage and one for current, were made on the topand the bottom of each of the photovoltaic cells. Transientphoto-excitation was used to avoid heating the silicon photovoltaiccells and to obtain JV curves under standard temperature conditions (25°C.). A flash lamp with a spectral output similar to the solar spectrumilluminated the photovoltaic cells from a vertical distance of 1 m. Thelamp power was held constant for 14 milliseconds. The intensity at thesample surface, as calibrated against external solar cells was 1000 W/m²(or 1 Sun) during this time period. During the 14 milliseconds, the JVtester varied an artificial electrical load on the sample from shortcircuit to open circuit, The JV tester recorded the light-inducedcurrent through, and the voltage across, the photovoltaic cells whilethe load changed over the stated range of loads. A power versus voltagecurve was obtained from this data by taking the product of the currenttimes the voltage at each voltage level. The maximum of the power versusvoltage curve was taken as the characteristic output power of the solarcell for calculating solar cell efficiency. This maximum power wasdivided by the area of the sample to obtain the maximum power density at1 Sun intensity. This was then divided by 1000 W/m² of the inputintensity to obtain the efficiency which is then multiplied by 100 topresent the result in percent efficiency. Other parameters of interestwere also obtained from this same current-voltage curve. One suchparameter is fill factor (FF) which is obtained by taking the ratio ofthe maximum power from the solar cell to the product of open circuitvoltage and short circuit current. The FF is defined as the ratio of themaximum power from the solar cell to the product of V_(oc) and I_(sc),multiplied by 100.

Paste Adhesion Measurements

Paste adhesion to the silicon wafer was performed for determiningstability and long-term durability of solar cell devices. As describedbelow, adhesion can be assessed by attaching the fired paste to a solderribbon, and then pulling the soldered tab and measuring the forcerequired at break. Fired pastes exceeding 2 N force at break aregenerally considered to meet the industry requirement. Test samples forthe adhesion measurements were printed and fired the same way asdetailed in ‘solar cell fabrication’ section, except that the sampleswere printed with three bus bars instead of grid lines and a bus barusing a screen on 8″×10″ frame (Sefar Inc., NJ) with 325-mesh wires of22.5 microns diameter at 30° angle and 20 microns thick emulsion. Thesamples were fired at maximum 935° C. temperature. The print bar of thefired test sample was 2 mm×20 mm, with 4 mm spacing between the bars.The back side of the fired test sample was glued onto an aluminasubstrate using a 2-part epoxy, Hardman® (Royal adhesives and sealants,CA), and cured for at least 15 min. A 2 mm wide, 3″ long, tinned-copperribbon (Sn/Pb/Ag in 62/36/2 ratio; Ulbrich Inc, Conn.) was cut andflattened, and then a thin layer of no-clean flux, 959T (Kester Inc,Ill.) was applied onto a 1″ long portion and dried for 15 min. The driedflux-coated portion was then placed on top of the fired bus bar. Heat at320° C. was applied with the solder rod for approximately 5 sec,resulting in the attachment of the ribbon to the fired paste. This stepwas repeated for the other two bus bars of the same sample. The adhesiontest was performed by pulling the soldered tab at a 90° angle andmeasuring the force required at break with Instron® Model 5569 (InstronInc, MA). The average force required to pull each tab was recorded. Twosamples for each paste were tested, for a total of 6 tab pulls per pastecomposition. The average adhesion data from two samples of each ofComparative Experiment A and Examples 2 and 3 are presented in Table 7.

Comparative Experiment A

One batch of thick film paste was made by mixing 132.75 g silver, 2 wt %Pb—Te—Li—O and 13.02 g organic medium in a plastic jar using a THINKY®ARE-310 mixer (THINKY Corp., Laguna Hills, Calif.) for 1 min at 2000rpm. This step was repeated two more times until a thoroughly mixedblend was obtained. The dispersed mixture was then blended with tripleroll mill (Charles Ross & Son Company, Floor Model, 4″×8″) at a 1 milgap for three passes at zero psi and three passes at 100 psi to obtain athick paste. The viscosity of the blended paste was adjusted withadditional 0.08 g of Texanol (Eastman Chemical Company, Tenn.) to obtaina printable paste. The viscosity of the final paste was measured using aBrookfield HADV-1 Prime Viscometer (Brookfield Engineering Labs, Inc.,Middleboro, Mass.) with the thermostatted small-sample adapter at about10 rpm and was found to be 310 Pas. The solid content of the final pastewas measured in duplicate by weighing small quantities (1-2 g) into analumina boat and firing in a muffle furnace at 450° C. for 30 mins toremove organics and weighing the alumina boat and contents. The averagesolid content of the paste determined to be 91.24 wt %.

The Pb—Te—Li—O was prepared as follows. Mixtures of TeO₂, powder (99+%purity), PbO powder (ACS reagent grade, 99+% purity) and Li₂CO₃ in the %cation ratio Te:Pb:Li of 57:38:5 were tumbled in a polyethylenecontainer for 30 min to mix the starting powders. The starting powdermixture was placed in a platinum crucible and heated in air at a heatingrate of 10° C./min to 900° C. and then held at 900° C. for one h to meltthe mixture. The melt was quenched from 900° C. by removing the platinumcrucible from the furnace and pouring the melt onto a stainless steelplaten. The resulting material was ground in a mortar and pestle to lessthan 100 mesh. The ground material was then ball-milled in apolyethylene container with zirconia balls and isopropyl alcohol untilthe average particle size (d₅₀) was 0.5-0.7 microns. The ball-milledmaterial was then separated from the milling balls, dried, and runthrough a 100 mesh screen to provide the PbO—TeO₂—Li₂O powder(Pb—Te—Li—O) used in the thick film paste preparations.

The organic medium components of the thick film paste and the quantitiesused are given in Table 5. The organic medium was prepared by mixing thecomponents.

TABLE 5 Component Wt. (g) 75% Gum Damar resin dissolved in aromatic 1500.75 solvent Ethyl Cellulose (50-52% ethoxyl) 1.11 Ethyl Cellulose(48-50% ethoxyl) 1.11 Duomeen, an amine oleate surfactant 0.75 Foralyn(hydrogenated rosin ester) 3.9 Thixatrol ST, hydrogenated castor oil0.75 DBE-3 (dibasic ester-3) 3.9 Texanol 0.75

The resulting paste was used in Comparative Experiment A. The paste wasprinted on the front side of a 28 mm×28 mm multicrystalline siliconwafer to form a front side electrode and then dried. A commerciallyavailable aluminum paste PV381 (DuPont Co., Wilmington, Del.) was screenprinted on the back side of the silicon wafer to form the back sideelectrode and then dried. Essentially identical additional wafers wereprepared in a similar fashion. These wafers were fired at four differentmaximum firing temperatures ranging from 900 to 960° C. to form thefront and back side electrodes of the solar cell. At least three to fivewafers were fired at each of the temperatures. The median solar cellefficiency and fill factor, measured as described above, are shown inTable 6 for the samples prepared at each of the maximum firingtemperatures. The adhesion results are shown in Table 7. Since the pastemade and used in this Comparative Experiment contained Ag but no Al₂O₃,it is designated as having a composition of 100 vol % Ag, based on thetotal volume of Ag and Al₂O₃.

Example 1

A second batch of thick film paste was made essentially as described inComparative Experiment A, except that the amount of Ag, i.e., the volumeof Ag, was replaced with a mixture of Ag and Al₂O₃ in which 50% of thatvolume was Ag and 50% was Al₂O₃. This paste is designated as having acomposition of 50 vol % Ag/50 vol % Al₂O₃, based on the total volume ofAg and Al₂O₃. Solar cells were made and measurements obtained asdescribed in Comparative Experiment A. The median solar cell efficiencyand fill factor, measured as described above, are shown in Table 6 forthe samples prepared at each of the maximum firing temperatures.

Example 2

A third batch of thick film paste was made by mixing equal amounts ofthe pastes made in Comparative Experiment A and Example 1. This resultedin a paste with a mixture of Ag and Al₂O₃ with 75 vol % Ag and 25 vol %Al₂O₃, based on the total volume of Ag and Al₂O₃. This paste isdesignated as having a composition of 75 vol % Ag/25 vol % Al₂O₃, basedon the total volume of Ag and Al₂O₃. Solar cells were made andmeasurements obtained as described in Comparative Experiment A. Themedian solar cell efficiency and fill factor, measured as describedabove, are shown in Table 6 for the samples prepared at each of themaximum firing temperatures. The adhesion results are shown in Table 7.

Example 3

A fourth batch of thick film paste was made by mixing equal amounts ofthe pastes made in Comparative Experiment A and Example 2. This resultedin a paste with a mixture of Ag and Al₂O₃ with 87.5 vol % Ag and 12.5vol % Al₂O₃, based on the total volume of Ag and Al₂O₃. This paste isdesignated as having a composition of 87.5 vol % Ag/12.5 vol % Al₂O₃,based on the total volume of Ag and Al₂O₃. Solar cells were made andmeasurements obtained as described in Comparative Experiment A. Themedian solar cell efficiency and fill factor, measured as describedabove, are shown in Table 6 for the samples prepared at each of themaximum firing temperatures. The adhesion results are shown in Table 7.

TABLE 6 Paste Maximum Efficiency Fill Factor Composition Firing Temp.(%) (%) 100 vol % Ag 905° C. 15.36 75.45 (Comparative 920° C. 15.57 77.8Experiment A) 935° C. 15.57 77.9 950° C. 15.80 78.3 87.5 vol % Ag/ 905°C. 15.53 77.85 12.5 vol % Al₂O₃ 920° C. 15.26 77.65 (Example 3) 935° C.15.37 76.45 950° C. 14.43 74.1 75 vol % Ag/ 905° C. 14.94 77.2 25 vol %Al₂O₃ 920° C. 15.72 77.45 (Example 2) 935° C. 15.27 77.45 950° C. 14.9475.3 50 vol % Ag/ 905° C. 14.16 71.5 50 vol % Al₂O₃ 920° C. 15.31 76.15(Example 1) 935° C. 15.25 76.1 950° C. 14.76 74.9

TABLE 7 Paste Average Composition Example # Adhesion (N) 100 vol % AgComparative Experiment A 0.93 87.5 vol % Ag/ Example 3 4.67 12.5 vol %Al₂O₃ 75 vol % Ag/ Example 2 3.45 25 vol % Al₂O₃

What is claimed is:
 1. A thick film silver paste comprising: (a) silver;(b) Al₂O₃; (c) a Pb—Te—O; and (d) organic medium; wherein said silver,said Al₂O₃, and said Pb—Te—O are dispersed in said organic medium,wherein said Al₂O₃ is present in an amount of 5-50 vol % based on thetotal volume of said silver and said Al₂O₃.
 2. The thick film silverpaste of claim 1, wherein the total content of said silver, said Al₂O₃and said Pb—Te—O is 50 to 95 wt %, based on the total weight of thepaste.
 3. The thick film silver paste of claim 2, said thick film pastecomprising 50-95 vol % silver, wherein said vol % is based on the totalvolume of said silver and said Al₂O₃.
 4. The thick film silver paste ofclaim 3, said thick film paste comprising 70-90 vol % silver and 10-30vol % Al₂O₃, wherein said vol % are based on the total volume of saidsilver and said Al₂O₃.
 5. The thick film silver paste of claim 3, saidthick film paste comprising 50-60 vol % silver and 40-50 vol % Al₂O₃,wherein said vol % are based on the total volume of said silver and saidAl₂O₃.
 6. The thick film silver paste of claim 1, said thick film pastecomprising 0.5-10 wt % Pb—Te—O, wherein said wt % is based on the totalweight of said thick film paste.
 7. The thick film silver paste of claim6, said Pb—Te—O comprising a Pb—Te—Li—O.
 8. The thick film silver pasteof claim 6, said Pb—Te—O comprising a Pb—Te—LI—Ti—O.
 9. The thick filmsilver paste of claim 6, said Pb—Te—O comprising a Pb—Te—B—O.
 10. Thethick film silver paste of claim 6, said thick film paste comprising0.5-0.5 wt % Pb—Te—O, wherein said wt % is based on the total weight ofsaid composition.
 11. The thick film silver paste of claim 10, saidPb—Te—O comprising a Pb—Te—Li—O.
 12. A solar cell comprising anelectrode formed from the thick film silver paste of any of claims 1-11,wherein said thick film paste has been fired to remove the organicmedium and form said electrode.
 13. A solar cell comprising an electrodeformed from the thick film silver paste comprising: a) silver; b) Al₂O₃;c) Pb—Te—O; and d) organic medium; wherein said silver, said Al₂O₃, andsaid Pb—Te—O are dispersed in said organic medium and wherein said thickfilm paste has been fired to remove the organic medium and form saidelectrode, wherein said Al₂O₃ is present in an amount of 5-50 vol %based on the total volume of said silver and said Al₂O₃.
 14. The solarcell of claim 13, said Pb—Te—O comprising a Pb—Te—Li—O.